This invention related generally to motion tracking and, in particular, to a system operative to optically monitor and record full-body and partial-body movements.
Numerous systems exist for measuring object surface or point locations by triangulation exist in the literature. The typical system projects a beam of collimated light onto an object and images that light through a sensor (typically a CCD) which is laterally displaced from the projector. The parallax displacement along the axis between the projector and the sensor can be used (along with the baseline between the sensor and projector) to compute range to the illuminated point.
Typical examples of this type of system include those described in U.S. Pat. No. 5,198,877 (Schulz), U.S. Pat. No. Re. 35,816 (Schulz), U.S. Pat. No. 5,828,770 (Leis et al.), U.S. Pat. No. 5,622,170 (Shulz), Fuch et al., Yamashita et al., and Mesqui et al. U.S. Pat. No. 5,198,877 (Schulz) and U.S. Pat. No. Re. 35,816 (Schulz) presents an optical tracking device that samples the three-dimensional surface of an object by scanning a narrow beam of light over the surface of an object and imaging the illuminated points from multiple linear photo detector arrays. The three-dimensional location illuminated is determined by triangulation (i.e. from the parallax displacement along each detector array of the illuminated spot). The system described also uses fixed but widely separated light sources as a calibration source. These light sources are time multiplexed so as to distinguish them from each other at the detect array. This system uses a cylindrical lens system to project light spot images onto the linear photo detector array.
U.S. Pat. No. 5,828,770 to Leis et al. presents a system for determining the spatial and angular orientation of an object in real-time based on activatable markers on the object imaged through two imaging sensors separated by a baseline. This system recognizes the light emitting markers based on geometrical knowledge from a marker-identification mode. Multiple markers are activated simultaneously and image together on the sensor focal planes. Mesqui, Kaeser, and Fischer (pp. 77-84) presents a system which is substantially the same as U.S. Pat. No. 5,828,770 except applied to mandible measurement and with some implementation details change.
U.S. Pat. No. 5,622,170 to Schulz describes a means for determining the position of the endpoint of an invasive probe inserted into a three dimensional body by locating two light emitting targets located at known locations on a portion of the probe still visible outside of the body. The means for tracking the light emitting markers is through imaging on three linear CCD sensors. This system uses a cylindrical lens system to project light spot images onto the linear CCD array.
Fuch, Duran, Johnson, and Kedem presents a system which scans laser light over a body and images the light spots through three cylindrical lenses and linear CCD cameras displaced in linear position and located out of plane from each other. Triangulation based on shift of the bright position along each CCD allows localization of the illuminated point on the body. Yamashita, Suzuki, Oshima, and Yamaguchi presents a system which is substantially the same as Fuch et al. except with respect to implementation details. Mesqui, Kaeser, and Fischer (pp. 52-57) is substantially the same as Fuchs et al. except that it uses only two linear CCD cameras instead of a photodiode array.
West and Clarke describe how to improve simple light spot detection algorithms which threshold the digitized signal from the imaging sensor and determine the spot location by averaging or taking the center of area of the pixels over the threshold. This paper describes a more accurate method which is used in the invention describe following that correlates a model of the illumination (or light spot) with the image. The correlation approach, by fitting the model to the image data, can provide a more accurate estimate of spot locationxe2x80x94typically 5 to 10 times better localization than would be possible through the simple thresholding approach. This method is important in three dimensional triangulation systems because small errors in spot location estimation on the imaging device translate into larger angular measurement errors and ultimately potentially very large errors in three-dimensional target location estimation.
The target locating systems described are used to track specific body points for medical purposes or proved the means for capturing object surface points for the purpose of three-dimensional digitization of object geometry. In all of the systems above targets are either projected from scanned collimated light sources or are active light emitting markers affixed to the object that is tracked. Several of the methods utilize linear CCD sensors that capture light through cylindrical lens systems. Some of the systems utilize more than one active emitter, but these emitters are distinguished from each other through geometrical market identification (not time multiplexing). None of these systems describe a tag or marker controller that is synchronized with the imaging sensor systems.
Broadly, this invention resides in an optical system capable of tracking the motion of objects, including the human body or portions thereof. This system provides for near simultaneous measurement of a plurality of three-dimensional active markers preferably affixed to the object or person to be tracked.
The system tracks active emitting markers through triangulation from data read via multiple linear CCDs through cylindrical lenses. The targets are identified with an improved method that resolves all need for geometrical identification. Each marker is lit in sequence so that it is in sync with a frame capture using the imaging system positioned and oriented so as to provide a basis for computing marker three dimensional location.
The system synchronizes the high-speed imaging of individual markers in the field via three synchronized linear CCD or photodiode arrays to localize position in three dimensions through triangulation techniques. In the preferred embodiment, the imaging system detects an infrared signal which is sent out by the tag controller as part of the tag/marker illumination sequence at the beginning of the first tag position capture time. The controller then traverses through the tags in time sync with each imaging system frame capture cycle. Thus, only one unique tag will be lit during each image capture of the cameras, thereby simplifying identification. Using linear CCD sensors, the frame time (i.e. point acquisition time) is very short, allowing very many markers to be sampled and located sequentially in real time.